Using the new Yocto-milliVolt-Rx for pH measures

By mvuilleu, in Measures and New stuff, june 27,2014.

After the Yocto-Volt and the Yocto-0-10V-Rx, we add today the Yocto-milliVolt-Rx to our family of electrically insulated voltage sensors. This new module is specifically designed for precision measures of low electric signals, from less than a millivolt to a maximum of 2V. Its design enables a direct measure of very fragile voltages, such as in a pH probe for instance.

Specificities of the Yocto-milliVolt-RX

There are two variants of the Yocto-milliVolt-Rx, differing only by the connector used for the measure: either a simple screw terminal, or a BNC connector to directly connect a shielded coaxial cable. Indeed, when you want to measure signals with a very low intensity, it might be important to protect the signal to be measured from surrounding electromagnetic interferences.

The two versions of the Yocto-milliVolt-Rx

Like its big brothers, the Yocto-milliVolt-Rx is an electrically insulated sensor. This not only enables you to measure voltages which do not share a common ground with the computer to which the sensor is connected, but it also provides you with an increased accuracy by avoiding electrical interferences due to untimely ground loops through the measuring wires (the 50Hz hum you typically hear in cheap audio appliances)

An essential specificity of the Yocto-milliVolt-Rx and of the Yocto-milliVolt-Rx-BNC, compared to other voltage sensors, is the use of an input operational amplifier with a very low current bias. To speak plainly, this means that the module is able to perform its voltage measure while diverting only a tiny part of the current of the observed phenomenon: less than 1 picoAmp, thus only a few electrons per microsecond...

Comparison

As numbers are often more meaningful than words, here is a short comparative table of our voltage sensors to date:

Yocto-Volt

Yocto-0-10V-Rx

Yocto-milliVolt-Rx

Measure range:

-250...250V DC0...250V AC

0...10V DC

-1V...2V DC

Sensitivity:

10mV

1 mV

up to 0.01 mV

Refresh rate:

10 Hz (DC)

50 Hz

100 Hz

Measure channels:

1

2

1

Input impedance:

331 K&ohm;

11.2 K&ohm;

> 1000 G&ohm;

Specificity

EmbeddedAC measure

Embedded 23V power supply

Very-highimpedance

Another specificity of the Yocto-milliVolt-Rx: similarly to the Yocto-0-10V-Rx, it is a generic sensor capable of performing implicitely a linear mapping between the measured signal (expressed in milliVolts) and a physical quantity, such as for instance a pH measure.

Sample application: a pH sensor

People asked us more than once if we intended to build a pH sensor. Now, a pH sensor is only the combination of a pH electrode and a high-sensitivity voltage sensor, such as the Yocto-milliVolt-Rx-BNC. We are not going to sell pH electrodes, this is not in our line, but we are nevertheless going to show you how to use a pH electrode with a Yocto-milliVolt-Rx-BNC.

Yocto-milliVolt-Rx-BNC + pH probe = USB pH meter

You can obtain your pH electrode (equipped with a BNC connector) from several serious vendors, for about $100. We also tested the $25 version bought on Aliexpress (on the Gain Express Holdings Ltd shop, in Hong Kong). It had acceptable results although its probe characteristics are somewhat at the limit.

Theoretically, at 25░C, an ideal pH probe provides a voltage difference of 59.16mV per pH degree, with a 0mV voltage for the neutral pH (7). So you only need to configure the Yocto-milliVolt-Rx-BNC to make the [-500mV..+500mV] range correspond to [15.45°pH...-1.45°pH] to transform it into an approximate pH-meter.

In real life, the pH electrodes have specific characteristics, in particular a reference voltage at the neutral pH which can vary up to +/- 30mV. If you want an accuracy above 0.5°pH, you need to use a neutral pH buffer solution to correctly center the pH range depending on the voltage. But this is not all: the voltage difference per °pH also decreases as the electrodes become older. In practice, what is generally done is a 2 point linear calibration, using two buffer solutions with distinct pH values. Let's have a look at how to proceed:

1. Measure the electrode voltage in each of the two buffer solutions, at ambient temperature. Make sure to rinse the electrode between the two solutions, and to allow enough time for the electrode output to stabilize.
- 1st measure: buffer solution with pH 10.01: -145 mV
- 2nd measure: buffer solution with pH 4.003: 195 mV

To ease your task, we will soon add in some of our API libraries a small app that will do these computations for you.

Here is the result in a few experiments:

If you want to learn more on pH electrodes, we found an excellent guide to pH measurement. In particular, it is also possible to compensate temperature differences to increase the accuracy even more (the error is of about 3/1000 per ░C). But in this domain, real electrodes also tend to diverge from the theory. The best advice is in general to use buffer solution at approximately the same temperature as the sample to be measured. It's the easiest and the most reliable solution :-)

Conclusion

One important detail: currently, there is no officially recommended enclosure for the Yocto-milliVolt-Rx-BNC. The device has been designed to fit in a YoctoBox-Long-Thick-Transp, but you have to manually bore a hole for the BNC connector in the side of the enclosure cover. As soon as we have created the appropriate tooling, we are going to offer a dedicated enclosure with a ready-made hole.

The Yocto-milliVolt-Rx-BNC fits in a long and thick enclosure, but you'll have to bore it.